runtime: fix function args not checked warning on arm

This tiny nit was driving me nuts

R=rsc, ken, r
CC=golang-dev
https://golang.org/cl/4550069

src/pkg/runtime/arm/closure.c

@@ -43,7 +43,7 @@ vars:		WORD	arg0
 		WORD	arg2
 */
 
-extern void cacheflush(byte* start, byte* end);
+extern void runtime·cacheflush(byte* start, byte* end);
 
 #pragma textflag 7
 void